The invention relates to a process for the preparation of isocyanates by reacting the appropriate amines with phosgene, condensing the gas mixture thereby obtained, stripping the liquid phase thereby obtained and returning the solvent so retained in liquid form to the reaction of amine with phosgene. The gaseous constituents are subsequently purified further in an absorption process.
The preparation of isocyanates has been sufficiently known from the prior art for a relatively long time, with phosgene generally being used in a stoichiometric excess, based on the amine or a mixture of two or more amines. Processes for the preparation of organic isocyanates from primary amines and phosgene are described in the literature, for example in Ullmann's Encyclopedia of Industrial Chemistry, 5th ed. Vol. A 19 p. 390 ff., VCH Verlagsgesellschaft mbH, Weinheim, 1991 and G. Oertel (Ed.) Polyurethane Handbook, 2nd Edition, Hanser Verlag, Munich, 1993, p. 60 ff. as well as in G. Wegener et al. Applied Catalysis A: General 221 (2001), p. 303-335, Elsevier Science B. V.
The synthesis of the phosgene used in the amine phosgenation is sufficiently known and is described, for example, in Ullmann's Enzyklopädie der industriellen Chemie, 3rd Edition, Volume 13, page 494-500. Further processes for the preparation of phosgene are described in, for example, in U.S. Pat. No. 4,764,308 and WO-A-03/072237. On an industrial scale, phosgene is mainly prepared by reacting carbon monoxide with chlorine, preferably on activated carbon as the catalyst. The strongly exothermic gas-phase reaction takes place at temperatures of from at least 250° C. to not more than 600° C., generally in tubular reactors. The heat of reaction can be dissipated in various ways. One way to dissipate the heat of reaction is, for example by means of a liquid heat-exchange agent such as those described in, for example, in WO-A-03/072237, or by vapor cooling via a secondary cooling circuit, while simultaneously using the heat of reaction to produce steam as disclosed in, for example, U.S. Pat. No. 4,764,308.
In the amine phosgenation, unreacted phosgene is mostly obtained at least partly in gas form together with the hydrogen chloride that is liberated. In the course of the working up of the isocyanate, phosgene and hydrogen chloride components that are still present in the liquid isocyanate-carrying product stream are separated off. In general, the product stream can still contain portions of solvent, inert gases, such as, for example, nitrogen and carbon monoxide, secondary products of the phosgene synthesis, such as, for example, carbon dioxide, and isocyanate which may have been carried along. In order to operate the isocyanate preparation process as economically as possible, it is essential to recover the excess phosgene with minimal losses and feed it to the phosgenation process again in a concentration, in the solvent in question, that is as optimal as possible for the process, as well as to separate off the hydrogen chloride gas obtained stoichiometrically and subject it to a suitable use, the particular uses making different demands on the hydrogen chloride in terms of purity.
Possible uses of hydrogen chloride include, for example, the sale of the aqueous solution (hydrochloric acid) or use of hydrochloric acid in other industrial or chemical processes. One of the most common possible uses of gaseous hydrogen chloride is the oxychlorination of ethylene with hydrogen chloride to give ethylene dichloride. Other preferred procedures include recycling processes for the hydrogen chloride and the return of the chlorine and/or hydrogen to the production process in which the hydrogen chloride is obtained. Such recycling processes include the catalytic oxidation of hydrogen chloride, for example according to the Deacon process, the electrolysis of gaseous hydrogen chloride and the electrolysis of an aqueous solution of hydrogen chloride (hydrochloric acid). WO-A-04/14845 discloses a process for catalytic oxidation according to the Deacon process, and WO-A-97/24320 discloses a process for the gas-phase electrolysis of hydrogen chloride. An overview of electrochemical recycling processes is given in the article “Chlorine Regeneration from Anhydrous Hydrogen” by Dennie Turin Mah, published in “12th International Forum Electrolysis in Chemical Industry—Clean and Efficient Processing Electrochemical Technology for Synthesis, Separation, Recycle and Environmental Improvement, Oct. 11-15, 1998, Sheraton Sand Key, Clearwater Beach, Fla.”.
The electrochemical oxidation of an aqueous solution of hydrogen chloride (hydrochloric acid) using a gas diffusion electrode as the cathode is described in WO-A-00/73538 and WO-A-02/18675.
In the electrolysis of aqueous hydrogen chloride according to the diaphragm or membrane process, the hydrochloric acid is used as electrolyte both in the anode chamber and in the cathode chamber. In the electrolysis, chlorine is produced at the anode and hydrogen at the cathode.
The mentioned possible uses of hydrogen chloride make specific demands in terms of purity, and accordingly, determine the purification outlay after separation of the majority of the other components from the phosgene/hydrogen chloride gas stream. Catalytic hydrogen chloride oxidation according to the Deacon process is carried out using a catalyst which requires prepurification of the hydrogen chloride gas from a phosgenation process by means of absorption on a purification bed or the catalytic combustion of solvent residues in the hydrogen chloride (see WO-A-04/014845). In the gas-phase electrolysis of hydrogen chloride using so-called solid electrolyte systems as described in WO-A-97/24320, contamination of the ion exchanger membrane or of the catalytically active material is not permitted, in order to avoid exchange of the units. In the electrochemical oxidation of an aqueous solution of hydrogen chloride using a gas diffusion electrode as cathode, WO-A-02/18675 proposes purifying the hydrochloric acid by means of activated carbon, and optionally, additionally by means of an exchanger resin. For the use of hydrogen chloride gas in oxychlorination, a two-stage condensation can be used to separate off troublesome impurities such as, for example, solvent residues (see U.S. Pat. No. 6,719,957).
An aqueous solution of hydrogen chloride (hydrochloric acid) for use in, for example, the foodstuffs sector requires a correspondingly high degree of purity. This high degree of purity can be achieved, for example, by adsorptive after-purification on an activated carbon bed, as is known from the prior art.
The treatment of the phosgene- and hydrogen-chloride-containing substance streams from the isocyanate preparation according to the prior art is described hereinbelow.
The general aim is to isolate the substance streams phosgene and hydrogen chloride, with secondary components contained therein, such as, for example, solvents, as economically as possible in the required purity, in order to be able to use phosgene or a phosgene solution of the desired concentration in the amine phosgenation again and to supply hydrogen chloride to a suitable use. The processes of condensation, partial condensation, washing, absorption, adsorption and distillation are conventionally used for that purpose, and are described, for example, in EP 1849797 A1.
A partial condensation of phosgene from the process gas can be achieved advantageously in terms of energy under elevated pressure, such as, for example from 10 to 50 bar, by means of cooling water. It is also reported that the solubility of the phosgene in the solvent is thereby increased, which results in an acceleration of the reaction. However, when this process is carried out on an industrial scale, the increased safety precautions with respect to a leakage resulting in the escape of phosgene are to be taken into consideration, as is described in DE-A-3212510.
Phosgenation reaction and working up of the gas phase under elevated pressure is also described in U.S. Pat. No. 3,544,611. The process gas is cooled with water in the range from 10 to 50 bar in order to condense a large part of the phosgene used in stoichiometric excess. A further phosgene depletion in the hydrogen chloride stream requires the use of refrigerating agents. A further economic advantage of amine phosgenation with working up of the process gas at elevated pressure is the saving in terms of refrigerating energy for the phosgene condensation, which allows the operation to be carried out in more concentrated solutions, and likewise constitutes an energy saving. Alternatively, U.S. Pat. No. 3,544,611 describes an embodiment in which hydrogen chloride from the process gas stream is condensed at 33 bar and a refrigerating agent temperature of −20° C. In a preliminary stage, phosgene is thereby condensed, while cooling with water, and separated off. The required purity of the two components is achieved by a distillation/stripping column between the two condensation stages. An inventive advantage of the present invention as compared with U.S. Pat. No. 3,544,611 is the production of a purified solvent stream in the process step of phosgene and hydrogen chloride separation and purification, which can be used directly for the preparation of a phosgene or amine solution.
In DE-A-10260084, reference is made to U.S. Pat. No. 3,544,611 in relation to an increased risk potential in the case of a leakage as a result of this pressurised procedure. It is further noted that, in the described processes, an undesirably high hydrogen chloride concentration is established in the phosgene for phosgenation, and phosgene is also lost with the hydrogen chloride stream (first variant). In the second variant, in addition to the reference of the already mentioned risk potential, reference is also made to the hydrogen chloride liquefaction at low temperatures and high pressures, which is disadvantageous in terms of energy. The hydrogen chloride must, in general, subsequently be evaporated again, with the use of energy, if it is to be further used.
In GB-A-827376, an amine phosgenation is carried out at about 3 bar. When the reaction is complete, excess phosgene and hydrogen chloride that has formed are separated off at the top of a column, at elevated temperature. From the gas phase, phosgene is condensed and the hydrogen chloride is relieved and conveyed away. With such a simple separation, large residual amounts of phosgene in the hydrogen chloride are to be expected, as well as undesirably high hydrogen chloride contents in the recovered phosgene, and therefore, also in the phosgene solution for the amine phosgenation.
Amine phosgenation to TDI and MDI in chlorobenzene is described in U.S. Pat. No. 3,381,025. When the reaction is complete, solvents are distilled off with phosgene and hydrogen chloride, then chlorobenzene and phosgene are condensed and fed to the phosgenation again, while hydrogen chloride, with not inconsiderable residual amounts of phosgene, is fed to phosgene destruction via an absorber. Here too, the phosgene/hydrogen chloride separation in both streams is incomplete, so that phosgene losses via hydrogen chloride result and undesirably large amounts of hydrogen chloride are contained in the phosgene, and ultimately in the phosgene solution, thus promoting disadvantageous amine hydrochloride formation in the amine phosgenation.
The amine phosgenation disclosed in SU-A-1811161 teaches that the phosgene- and hydrogen-chloride-containing process gas stream separated from the liquid solvent/isocyanate product stream still contains 4% phosgene in the hydrogen chloride gas after several condensation and absorption steps, and is drawn off for further purification and use. This gas stream is combined with the waste gas from a phosgene absorber, in which gas streams from the phosgene preparation and streams from the phosgenation that cannot be condensed further are treated. This waste gas stream from the absorber is described as having a content of 4% chlorine. The chlorine and phosgene impurities mentioned in these partial streams indicate that the phosgene preparation process is disadvantageous and that, in accordance with an incomplete phosgene/hydrogen chloride separation, the phosgenation process is capable of being improved. A process step for minimising the circulating solvent (chlorobenzene), analogous to the present publication, is not described.
In EP-A-0570799, a publication relating to amine phosgenation in the gas phase, reference is made to the separation of excess phosgene after condensation of the prepared isocyanate in a manner known per se. This can be effected by means of a cold trap, absorption in an inert solvent (e.g. chlorobenzene or dichlorobenzene) maintained at a temperature of from −10° C. to 8° C., or by adsorption and hydrolysis on activated carbon. The latter variant appears to have no economic value when carried out on a large scale. The hydrogen chloride gas that flows through the phosgene recovery stage can be recycled in a manner known per se in order to recover the chlorine required for the phosgene synthesis.
U.S. Pat. No. 3,226,410 describes a continuous two-stage amine phosgenation process in the liquid phase. A phosgene solution is mixed in stoichiometric excess with an amine solution in a tubular reactor at temperatures up to 90° C. The second stage takes place in a boiler at from 110 to 135° C. The gas phase, consisting of phosgene, hydrogen chloride and solvent portions, is removed at the top of the second stage, condensed in two stages and fed to the phosgene solution container. Non-condensable portions pass into an absorption column, where phosgene still present in the gas stream is absorbed by means of distilled solvent from the liquid phase of the phosgenation and is fed to the phosgene solution container. Portions from the absorption column that have not been absorbed, for the most part hydrogen chloride gas, are fed to a water-operated HCl absorber in which aqueous hydrochloric acid is produced. In contrast to the present invention, U.S. Pat. No. 3,226,410 does not describe in the amine phosgenation, in the phosgene solvent circuit, a process step in which the amount of solvent is controlled, or optimised or minimised, relative to the total amount of phosgene solution.
A chemical separation of hydrogen chloride and phosgene is of lesser importance for large-scale isocyanate preparation, in view of the treatment and working up of the substance streams solvent, phosgene and hydrogen chloride, because of the large amount of bases, for example, that is used, the loss of hydrogen chloride and the large amount of secondary products formed. In EP-A-1020435 and DE-A-1233854, for example, tertiary amines are used as hydrogen chloride acceptors, which are obtained as solids in the form of the hydrochloride. Alkali or alkaline earth salts or oxides are used for this purpose in JP-A-09208589.
The aim of DE-A-10260084 is to obtain hydrogen chloride of maximum purity and pure phosgene from a substance mixture as is conventionally formed in the preparation of isocyanates by reaction of amines with phosgene. This reference describes a process which in principle has four stages, the fundamental stages of which require two separate columns as well as additional units. The process gas from the isocyanate preparation consists predominantly of phosgene, hydrogen chloride, solvent portions and also low boilers and inert substances, carbon monoxide and carbon dioxide being mentioned here as examples. The first process step is the partial condensation of the process gas, which can take place in one or more stages, it being possible to work, depending on the installation pressure, at from 40° C. by means of cooling water to −40° C. with cooling with brine. The partially condensed mixture so obtained is then guided between the stripping part and the rectifying part into the downstream distillation column. In the indicated example with chlorobenzene as solvent, this column is in the form of a bubble tray column with 22 trays in the stripping part and 11 trays in the rectifying part. The column is used to remove hydrogen chloride from the phosgene and is equipped for that purpose with a circulation evaporator (Robert evaporator) and a tubular heat exchanger as top condenser. At a supply temperature of 24.5° C., a bottom temperature of 38° C., a temperature at the top of −9° C. and a pressure at the top of 2.5 bar, the reflux temperature of the partial condensation product at the top of the column is −20° C. Under these conditions, the product removed at the bottom is indicated as having a hydrogen chloride content of 0.01 wt. %, phosgene is given as 89 wt. % and chlorobenzene as 10 wt. %. This stream is fed to the reaction part of the isocyanate synthesis.
Alternatively to the mentioned evaporator in the distillation column, removal of hydrogen chloride can also be carried out by stripping with an inert gas such as nitrogen, the process solvent vapor, phosgene or another gaseous substance or substance to be evaporated from the process waste gas stream that is to be treated.
The portion not condensed in the top condenser of the distillation column, consisting of 74 wt. % hydrogen chloride and 26 wt. % phosgene, is passed at −20° C. into the lower region of an absorption column which is equipped with three charges of wire mesh rings. Chlorobenzene at −25° C. is introduced at the top of the washer, and the heat of solution of the hydrogen chloride in chlorobenzene is dissipated by an intermediate cooler operated at −30° C. At the top of the washer there are obtained vapors which are fed, downstream of a demister, to a top condenser operated at −30° C. Droplets are hereby retained which, together with a small amount of condensed vapors, are fed into the bottom of that absorber or washer. The top of the column is operated at 2.2 bar and −8° C. and the bottom at 6° C. The product removed at the top, downstream of the condenser, has a hydrogen chloride content of 99.5 wt. %, a phosgene content of 0.1 wt. % and a chlorobenzene content likewise of 0.1 wt. %. The product discharged at the bottom contains 19 wt. % phosgene, 78 wt. % chlorobenzene and 3 wt. % hydrogen chloride.
In the example of DE-A-10260084, the gaseous product discharged at the top is after-purified with an activated carbon filter without its being possible to detect phosgene or chlorobenzene residues by GC analysis or IR spectroscopy.
According to the invention, the product discharged at the bottom of the absorber, with the above-mentioned contents of phosgene and hydrogen chloride, is to be fed to a reaction column, a column for phosgene separation or as reflux for working up of the reaction mixture. In the latter case, reference is made to the possibility of not using a vapor condenser for producing the reflux.
DE 10260084 A1 claims the separation and recovery of hydrogen chloride and phosgene or phosgene solution for re-use or further processing, starting from a substance mixture as is obtained in an amine phosgenation. An optimisation of or increase in the phosgene concentration in the phosgene solution, achieved by the process claimed in this invention or a comparable technology without a marked increase in the installation pressure, is not described.
Starting from this prior art, one technical object has been to provide a process for the preparation of isocyanates, which process comprises purifying the stream obtained thereby, containing hydrogen chloride, phosgene and low boilers and inert substances, and with which solvent, phosgene and hydrogen chloride can be obtained again simply and economically with high purities. At the same time, the technical object has been to increase, in a simple and economical manner, the concentration of phosgene in the phosgene solution in the bottom of the absorption and concentration unit (see examples in EP 1 849 767 A1, referred to hereinbelow as absorber for short) at the same pressure and at the same temperature, or to increase further the hydrogen chloride purity of the hydrogen chloride obtained at the top of the absorber, while the concentration of phosgene in the phosgene solution in the bottom of the absorber remains the same, by increasing the amount of washing liquid supplied.
WO 2006/029788 A1 describes a process for separating hydrogen chloride and phosgene, with the required purity of the individual substances, using ionic solvents. In terms of possible savings in the energy field, this method has the disadvantage that, for example, more than one solvent is required in an amine phosgenation process.
As compared with the processes of the prior art, the process according to the invention provides the possibility of reducing the amount of solvent required in the dephosgenated isocyanate stream, which must be separated off during the working up to give the solvent-free isocyanate, by means of a higher phosgene concentration in the solvent carried in the circuit of the process as a whole. This process provides a marked advantage in terms of energy. Alternatively, while the total amount of solvent in the circuit remains the same, the amount of solvent for purifying the hydrogen chloride can be increased further if an appropriately chosen subsequent process so requires. It has now been found that this can be achieved by the process according to the invention.